1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) and in particular to improvement of display quality of a vertically aligned LCD having an alignment control window.
2. Description of the Related Art
A vertically aligned LCD having an alignment control window is proposed, for example, in Japanese Patent Laid-open No. Hei 6-301036, in which alignment direction of liquid crystal is controlled. The vertically aligned LCD has liquid crystal having negative dielectric constant anisotropy and a vertically aligned film. In the following, an LCD of this type will be described.
FIG. 6A is a plan view showing an LCD having an alignment control window, and FIG. 6B is a cross sectional view of the LCD along the line A-Axe2x80x2 in FIG. 6A. A gate line 51 is formed on a first substrate (TFT substrate) 50, and covered by a gate insulating film 52. On the insulating film 52 there is formed a thin film transistor (TFT) 53 comprising a polysilicon film. A part of the gate line 51 constitutes a gate electrode for the TFT 53. Covering the above, an interlayer insulating film 54 is formed, on which a pixel electrode 55, comprising indium tin oxide (ITO), is formed being connected to the TFT 53 via a contact hole formed on the interlayer insulating film 54. On the pixel electrode 55 there is formed a vertically aligned film 56, comprising an organic material, such as polyimide, or an inorganic material, such as silane material. The interlayer insulating film 54 consists of two layers, including an interlayer insulating film 54a having a data line 57 formed thereon. The data line 57, connected to a source region of the TFT 53, supplies charges to the pixel electrode 55 when the gate line 51 is turned on. Note that the data line 57 is formed below the pixel electrode 55 in order to prevent the liquid crystal from being caused to slant directly by the voltage applied to the data line 57.
A second substrate 60 is located as it is opposing the first substrate 50, having a color filter 61 formed thereon at a position corresponding to the pixel electrode 55. Further, a common electrode 63, comprising ITO or the like, is formed via the insulating film 62 on the second substrate 60, opposing the plurality of pixel electrodes 55. Similar to the first substrate 50 side, a vertically aligned film 64 is formed on the common electrode 63. The common electrode 63 has an alignment control window 65 formed thereon at a position corresponding to the pixel electrode 55. An alignment control window 65, or an opening on the common electrode in absence of an electrode, has a shape resembling, for example, a xe2x80x9cYxe2x80x9d and a inverted xe2x80x9cYxe2x80x9d connected together, specifically, having branching upper and lower ends.
Liquid crystal 70 is enclosed between the first and second substrates 50, 60. Direction, or alignment, of the liquid crystal molecules is controlled according to the strength of an electric field formed by the voltage applied to between the pixel electrode 55 and the common electrode 63. Polarizers (not shown) are respectively provided on the outside of the first and second substrates 50, 60, with the polarizing axes thereof being vertical. A straight polarization (light) proceeding from one polarizer to the other is modulated while passing through the liquid crystal 70, to thus achieve a desired transmittance, the liquid crystal 70 being controlled for every display pixel to have an alignment corresponding to the applied voltage to the pixel electrode.
The liquid crystal 70 has negative dielectric constant anisotropy, and slants, by nature, relative to the direction of an electric field. The liquid crystal 70 is controlled to have vertical initial alignment by vertically aligned films 56, 64. In this case, with no voltage applied, the liquid crystal molecules are aligned vertically with respect to the vertically aligned films 56, 64. Therefore, the straight polarization having gone through the polarization panel on one side, proceeds through the liquid crystal layer 70 to reach and be shielded by the polarization panel on the other side, as a result of which a black is display is shown. When a voltage is applied to between the pixel electrode 55 and the common electrode 63 in the above structure, electric fields 66, 67 are generated, whereby the liquid crystal molecules are caused to slant. In particular, around the edges of each pixel electrode 55, the caused electric field 66 is vertical directing from the pixel electrode 55 toward the common electrode 63. Similarly, around the edges of each alignment control window 65, the caused electric field 67 is vertical directing toward the pixel electrode 55 due to the absence of an electrode. As the liquid crystal molecules are controlled to be aligned vertically with respect to this vertical electric fields, the molecules are resultantly forced to slant directing toward the inner side of each pixel electrode 55, i.e., the alignment control window 65. As a result, the straight polarization having penetrated the polarizer on one side is subjected to birefringence in the liquid crystal layer 70 to be thereby converted into elliptically polarized light before passing through the polarizer on the other side. With the above, a nearly white color is displayed.
The pixel electrode 55 is supplied with a voltage via the TFT, when the gate line 51 and the data line 57 are both turned on to drive the liquid crystal immediately thereabove. Application of appropriate voltages to the respective pixel electrodes 55 could achieve LCD display. That is, an area with a pixel electrode 55 formed thereon constitutes a pixel.
In a part immediately below the alignment control window 65, an electric field is not caused despite voltage application due to the lack of common electrode 63, and liquid crystal molecules in that part therefore remain in the initial state, or fixed directing in a vertical direction. As a result of the above, liquid crystal molecules on both sides of the alignment control window 65 have opposite alignments due to continuity of liquid crystal, and this contributes to ensuring a wider viewing angle.
An LCD voltage application method will next be described.
FIG. 7 is a timing chart for voltages to be applied to the gate lines 51 and the data line 57, and for a voltage of a pixel electrode driven thereby. (a), (b), and (c) show voltages to be applied to the first gate line 51, the second gate line 51, and the data line 57, respectively, the second gate line 51 being positioned next to the first gate line 51. (d) and (e) show voltages of the pixel electrode 55 to be controlled by the first gate line 51 and the data line 57, and of the pixel electrode 55 to be controlled by the second gate line 51 and the data line 57.
A voltage is applied to the first gate line 51 during one horizontal synchronization period (hereinafter denoted as 1H), so that the gate line 51 is turned on to thereby turn on the TFTs 53 of the associated arrayed pixel electrode 55 on the first horizontal line. In addition, voltages corresponding to a display image are continuously applied to the respective data lines 57 during a period 1H, so that the arrayed pixel electrodes 55 with turned-on TFTs can retain their voltages. During the next period 1H, the first gate line 51 is turned off, while the second gate line 51 is turned on. Accordingly, the TFTs of the pixel electrodes 55 corresponding to the second gate line 52 are turned on, and the pixel electrodes 55 connected to the TFTs on the second horizontal line can retain their voltages supplied from the data line 57. Subsequently, a voltage is supplied to pixel electrodes 55 in each row for every period 1H to thereby drive associated liquid crystal for image display. Here, an electric field is caused in an opposite direction for every pixel electrode row in order to prevent deterioration of the liquid crystal. Specifically, arrayed pixel electrodes 55 subject to control by the first gate line 51 are supplied with a voltage Vhigh (10V) higher than a potential Vc (e.g., 6V) of the common electrodes 63 by a predetermined potential (e.g., 4V), and the adjacent arrayed pixel electrodes 55 are supplied with an inverted voltage, or a voltage Vlow (e.g., 2V) lower than the potential Vc by a predetermined potential (e.g., 4V). Subsequently, the arrayed pixel electrodes 55 associated with the first gate line 51 are then supplied with a voltage inverted from that which was last supplied thereto, or a voltage Vlow. The above voltage application method is referred to as a line inversion method. According to this line inversion method, in which voltages for application to pixel electrodes are inverted using the voltage Vc of the common electrode 63 as a middle voltage value, electric fields of identical formations but in opposite directions are formed for every pixel electrode array in a horizontal line. Note that a voltage is applied to each pixel electrode 55 every time the corresponding TFT is on in the above example. Whether or not to apply a voltage different from that of the common electrode 63 to each pixel electrode 55 is determined according to display data.
Generally, an LCD of the above described type, i.e., a vertically aligned LCD having an alignment control window, can attain only weak control over the alignment direction of liquid crystal molecules, compared to an LCD of a rubbing-type, in which the alignment direction is controlled through rubbing. Therefore, problems may occur, for example, in that inconsistent distribution of spacers for defining the thickness of the liquid crystal layer 70 may result in inconsistent thickness of the liquid crystal layer 70, and that the alignment direction of the liquid crystal molecules may be disturbed by any alignment disturbing factor, such as an electric field applied from the outside, (hereinafter referred to outside disturbance), causing the characteristics of a viewing angle of a pixel to change. Because of the continuity of the liquid crystal, disordered alignment in a part of liquid crystal may affect the alignment direction of the remaining liquid crystal in a pixel. A boundary may be caused anywhere (not fixed) between the disorderly aligned liquid crystal and the properly aligned liquid crystal, constituting a plane with discontinuous liquid crystal alignment, or disclination. This may lead to a drop of an aperture ratio as no light passes through an area with disclination. In addition, outside disturbance, which may disturb the liquid crystal alignment differently for every pixel, may also cause uneven screen display with deteriorated LCD display quality.
Still further, when a glass substrate is charged, for example, positive due to outside disturbance, an area opposite thereto will be charged opposite, or negative. Although the charging may have less effect on a common electrode as it receives a voltage, an alignment control window, in absence of an electrode and an applied voltage, remains charged. A charged alignment control window may cause an unintended electric field, which may affect the alignment direction of the liquid crystal molecules within the pixel. In a macro view, spots with different colors may be caused. Also, the charging itself may be outside disturbance.
The present invention aims to provide a vertically aligned LCD having an alignment control window, which can attain high display quality.
According to the present invention, there is provided a vertically aligned LCD having an alignment control window formed by making an opening on a common electrode which is formed on a second substrate at a position corresponding to a pixel electrode, in which an alignment control assistance electrode is provided between the common electrode and the second substrate. With this arrangement, liquid crystal alignment direction can be more strongly controlled to remain more stabilized and less vulnerable to outside disturbance, such as an outside electric field. This contributes to improvement of LCD display quality.
According to another aspect of the present invention, when a line inversion method is employed as a driving method, it is preferable that the alignment control assistance electrodes are supplied with a voltage when a voltage is applied to the arrayed pixel electrodes adjacent to the opposing pixel electrodes. With this arrangement, no specific control circuit is required in applying a voltage to the alignment control assistance electrode, the voltage being inverted from that to be applied to the associated pixel electrode.